Some dual graphics machines support muxing the DDC separately from the
display, so make use of this functionality when reading the EDID on the
inactive GPU. Also serialize drm_get_edid() with a mutex to avoid races
on the DDC mux state.

Signed-off-by: Seth Forshee <seth.fors...@canonical.com>
---
 drivers/gpu/drm/drm_edid.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index a8743c3..1a4b661 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -31,6 +31,7 @@
 #include <linux/slab.h>
 #include <linux/i2c.h>
 #include <linux/module.h>
+#include <linux/vga_switcheroo.h>
 #include "drmP.h"
 #include "drm_edid.h"
 #include "drm_edid_modes.h"
@@ -82,6 +83,8 @@ struct detailed_mode_closure {
 #define LEVEL_GTF2     2
 #define LEVEL_CVT      3
 
+static DEFINE_MUTEX(drm_edid_mutex);
+
 static struct edid_quirk {
        char vendor[4];
        int product_id;
@@ -395,12 +398,26 @@ struct edid *drm_get_edid(struct drm_connector *connector,
                          struct i2c_adapter *adapter)
 {
        struct edid *edid = NULL;
+       struct pci_dev *pdev = connector->dev->pdev;
+       struct pci_dev *active_pdev = NULL;
+
+       mutex_lock(&drm_edid_mutex);
+
+       if (pdev) {
+               active_pdev = vga_switcheroo_get_active_client();
+               if (active_pdev != pdev)
+                       vga_switcheroo_switch_ddc(pdev);
+       }
 
        if (drm_probe_ddc(adapter))
                edid = (struct edid *)drm_do_get_edid(connector, adapter);
 
+       if (active_pdev && active_pdev != pdev)
+               vga_switcheroo_switch_ddc(active_pdev);
+
        connector->display_info.raw_edid = (char *)edid;
 
+       mutex_unlock(&drm_edid_mutex);
        return edid;
 
 }
-- 
1.7.9.5

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